Disulphobenzolic acid, or the dioxybenzol formed from it on melting-pyrocatechin according to Baeyer's researchesshould be found in the mother-liquors of alizarin.

This formation of phthalic acid, however, only occurs if the preparation of the sulpho-acids is not properly conducted. Where the experiment is successful no phthalic acid appears.

Monosulphanthraquinonic Acid, C11H,(SO2H)O2.

The pure acid is obtained from the barium or lead salt by accurate precipitation with sulphuric acid and concentration of the solution, when it separates in the form of yellow leaflets. A small quantity of sulphuric acid present in the solution is not injurious, as it diminishes the solubility of the compound in water. In hot water it deliquesces; in cold it dissolves more sparingly, but still readily. From a saturated solution it is thrown down on the addition of sulphuric or hydrochloric acid. Its salts are somewhat sparingly soluble in water.

Sodium Salt, C1H,O,(SO, Na).

It forms fine silvery leaflets, which dissolve in water with a yellow colour. In hot water it dissolves more freely, but sparingly in cold.

100 parts water at 100° C. dissolve 18.88 parts,

It is much more soluble in water than the baryta salt, but not much more plentifully at 100° C. than in the cold. It forms only indistinctly developed crystals.

Baryta Salt, [C1,H,(O2) SO2],Ba.

It forms indistinctly developed yellow crystals, which,

under the microscope, appear as tables. In cold water it is very sparingly soluble; more freely, but still slightly, at a The addition of hydrochloric acid diminishes the

boil. solubility.

If the potassium or sodium salt of monosulphanthraquinonic acid is heated with caustic potassa or soda, the colour, which is at first red, passes into violet. If the heat is strong enough and sufficiently prolonged, the aqueous solution has the characteristic properties of alizarate of potassa, and acids precipitate alizarin from the solution. If the heat is lower, and if less alkali is used, or if indifferent salts, e.g. elutriated chalk, be added, the resulting solution is rather red than violet, and acids cause the separation of oxyanthraquinon which will be described below. On careful melting, the two successive reactions may easily be observed. There is first formed monooxyanthraquinon, the hydroxyl compound corresponding to monosulphanthraquinonic acid. On further fusion with potassa this is converted by its oxidising influence into bioxyanthraquinon-i.e. alizarin. The true reactions cannot, however, be sharply discriminated. The following equations explain the process :

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C1H,(O2)SO2K+2 KOH=C1,H,(O2)OK+K2SO ̧+ H2O, Oxyanthraquinon potassa.

C1H7(02)OK + KOH=C12H¿(O2) (OK)2+H2,

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Alizarin potassa.

Free hydrogen, however, does not seem to appear. It either exerts a reducing action upon the oxyanthraquinon or upon the sulpho-acid not yet attacked and forms anthraquinon, which is often found in alizarin, or it reduces the alizarin to hydroalizarin, which then during fusion or during solution in water is reoxidised to alizarin by the oxygen of the air.

Bisulphanthraquinonic Acid, CH。(O2) (SO2H)2.

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This acid is obtained like the above-described mono-acid, and separates out on evaporating the aqueous solution in yellow crystals much more soluble than those of monosulphanthraquinonic acid. In sulphuric acid it is sparingly soluble, and the mixture of anthraquinon and sulphuric acid used in its preparation congeals therefore on cooling. Its salts also are more soluble than those of the mono-acid. The barytic salt is among the least soluble, whence, independently of the cost, it is more suitable to use the lime salt for separating the product from the excess of sulphuric acid. The salts in the solid state are light yellow, but a yellowish red when in aqueous solution. The solutions of the alkaline compounds are redder than those of the other metals.

Bisulphanthraquinonic acid is formed also on heating bibrom- and bichlor-anthracen with sulphuric acid, which effects at once the formation of the sulpho-acid and the replacement of the atoms of bromine and chlorine of oxygen. At the same time there are formed bromiferous and chloriferous intermediate products, which have been subjected to a closer examination by Perkin. On treating bibrom- and bichlor-anthracen with sulphuric acid, sulpho-acids are formed even in the cold. Concerning these, Perkin has shown that they have the composition of bisulphobibromanthracenic acid, CH,Br,(SO,H)2, and of bisulphobichloranthracenic acid, CH.Cl2(SO,H)2. These are converted by the action. of sulphuric acid into bisulphanthraquinonic acid, the chlorine escaping as hydrochloric acid, and the bromine in the free state. The equations showing the formation of bisulphanthraquinonic acid are therefore best resolved into groups of two:

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For bichloranthracen we have,

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2

3

C1,H,Cl2+2 SH2O, C1,H,Cl2 (SO,H)2+2 H2O. C1H ̧Cl2(SO2H),+SH2O1=C12H2(O2)(SO2H)2+2 HC1-+ SO2.

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H

The following equations correspond to bibromanthracen :

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C11H,Br2+2 SH2O1=C12H ̧Br2(SO2H)2+2 H2O.

2

14

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C1H.Br2(SO2H)2+2 SH2O1 =С14H6(O2) (SO2H)2+2 Br+ 2 SO2+2 H2O.

The bromine and chlorine compounds dissolve in cold sulphuric acid, at first with a splendid green colour, which in a few seconds becomes a fine magenta. Water, however, precipitates the bromine and chlorine compounds unchanged. On the application of heat the colour disappears, the bromine compound emits red vapours of bromine, the chlorine compound gives off fumes of hydrochloric acid, and the solution then contains bisulphoanthraquinonic acid.

Another method for preparing bisulphoanthraquinonic acid sets out with first preparing bisulphoanthracenic acid, and subsequently oxidising. To obtain it by this method, 1 part by weight of anthracen is heated for some hours to 100° C. with 4 parts by weight of sulphuric acid of spec. grav. 1.848. The temperature is raised to 150°, and kept up for an hour. When cool it is diluted with 3 parts of water, and to 1 part of the anthracen used 2 to 3 parts of manganese are added, and the whole boiled for some time. To complete the process the mixture is concentrated or even evaporated to dryness. The sulphuric acid is neutralised with lime, an excess of which is used to precipitate the manganese, and carbonate of potassa or soda is added to the filtrate until all the lime is precipitated. In place of manganese other oxidising agents may be used, such as peroxide of lead, chromic acid, or nitric acid. If chromic acid is employed, the portion of it not reduced must be converted into chromic oxide before the addition of the lime by means of sulphurous acid. If nitric acid is used, the oxidising mixture is concentrated until the greatest part of the nitric acid is driven off.

C. Liebermann prepared disulphanthraquinonic acid from B-benzoyl-benzoic acid. If the latter is heated for some time with sulphuric acid till water no longer produces a precipitate, it is converted into bisulphanthraquinonic acid. The reaction takes place according to the equations—

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32

The baryta salt, C1H(O2) [SO,],Ba, is obtained from water in imperfectly developed yellow crystals, and dissolves sparingly in cold water, but rather more freely at a boil.

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The lead salt, C11H6(O2)[SO3]2Pb, forms, like the baryta salt, imperfectly developed yellow crystals, and dissolves slightly in cold but more freely in boiling water.

Bisulphanthraquinonic acid, if heated with hydrate of potassa or soda, yields isopurpurin; but here also an intermediate compound may be observed, and indeed much more easily than in the monosulphuric acid. At first only one of the residues of sulphuric acid is replaced by hydroxyl, and there is formed the potassium salt of a sulphoanthraquinonic acid:

On further heating, the other residue of sulphuric acid is substituted; but at the same time a hydrogen is replaced by a hydroxyl, forming a mixture of dihydroxylanthraquinon and isopurpurin. If the action of the hydrate of potassa is sufficiently prolonged, most of the dihydroxylanthraquinon is converted into isopurpurin, of which the final product chiefly consists, with a mere admixture of dihydroxanthraquinon. Which of the three products hitherto found in isopurpurini.e. in alizarin for reds—namely, anthraflavic acid, isoanthraflavic acid, anthraxanthic acid, is derived from the ordinary disulphanthraquinonic acid is not demonstrated with cer